1. Field of the Invention
The invention relates to water-dilutable urethane resins, processes for the preparation thereof, and to their use, for example, as binders for coating materials which can be crosslinked by free-radical polymerization.
2. Description of Related Art
Water-soluble or dispersible polyurethanes which contain carboxyl groups in the form of structural units of dimethylolpropionic acid and which are suitable as coating materials for substrates are known, for example, from EP 0 269 972 A2.
The preparation of similarly structured oligourethanes, which can be cured by free-radical polymerization, is described in EP 0 453 838 A2. Here, a mixture of alcohols containing (meth)acryloyl groups and dimethylolpropionic acid is reacted with diisocyanates in the presence of an organic solvent, such as butyl acetate, and the reaction product is converted by means of ammonia into the water-dilutable form.
Oligourethanes of this kind have significant disadvantages. For example, prior to dispersion in water, the products are from 60 to 95% strength solutions in organic solvents. Only part of the organic solvent can be removed, since the viscosity of the resins increases rapidly as the solids content rises. Moreover, the use of ammonia as a neutralizing agent for water-dilutable coating materials is being rejected more and more all the time.